Currently, in a marine application, a seismic source with a constant predefined directivity, is applied for shooting direction following a regular pattern at the sea surface, such as dip and strike straight line, straight line star and multi azimuth, circular and elliptical surveys. A source directivity angle greater than the critical angle will generate overcritical secondary energy and reduced primary energy, whereas a source directivity angle lower than the critical angle will generate a reduced illumination sector for the highest angles. Ideally, the source directivity is designed to be close to the critical angles for the boundaries located at and close to the seafloor, to avoid overcritical energy and reduced illumination. Normally, the source directivity is adjusted for younger and relatively low velocity, high critical angle strata in the overburden. The corresponding source directivity optimal for the relatively high critical angle boundaries in this overburden is therefore relatively open. During acquisition of a conventional survey, the source directivity is kept constant and often the same source directivity is applied for surveys in different geological provinces.
Due to the uplift and erosion history in the Barents Sea, older and relatively high velocity, low critical angle boundaries are located close to the seafloor. This situation is extreme in the Nordkapp Basin characterised by the presence of several high velocity salt diapirs located close to the seafloor. The older sediments surrounding the salt diapirs here tend to dip steeply, creating a high velocity gradient towards the salt and along the unconformity boundary situated close to the seafloor. In this area, the near seafloor boundaries will generally have very variable critical angles, between a normal critical angle in the younger strata away from the salt to very low values for the top salt boundary.
Application of a normal and constant directivity source, for instance over basalts or salt, will therefore generate over-critical energy of a varying degree and with maxima above the top high velocity layer.
The effects of overcritical generated noise are assumed to be the origin of the noisy regions with lack of primary events, defined as the shadow zone, beneath and surrounding the high velocity layer.
Many attempts have been made to remove the source-generated noise by processing, but these have so far been unsuccessful. When the target is within a shadow zone, it is of great interest to improve the data quality for further successful exploration of this basin.